I still think there are many great use cases for N900, just not as a linux phone, browser, navigation.

Mypaint with stylus is still fun.
It is a good device for growing stuff, you can monitor your plants with camera measure light intensivity, control a pump via usb power (5v) or sd card power (3.3) or vibration motors contacts and even play music to them (which surprisingly helps them)
turn it into shower radio, resistive touchscreen works even through a shopping bag..
hackable bluetooth keyboard (dont try bluetooth is too ****ed up on n900 )
or a usb boot device where you can select bootable system via n900 touchscreen and mount your system partition readonly (very useless but very secure )
[if you look at the n900 itself the broadband cpu was used as main cpu in older modells, so its kind of the natural process to let your old phone act as io slave/device (some people use their old pc/laptops as servers) (n900 lack of ram makes this a bit difficult)]
use it as eyes for your robot
if i remember correctly there was a sound radar system based of a n900..

My advice
lost of fun stuff but dont engage if you are new and dont have tons of time to waste (hunting old hardware bugs that you maybe cant fix)

(i forgot if you focused the camera via gstreamer 0.10 (which nobody wants to support anymore) there was a bug that you werent allowed to focus too fast or you would get a knacking sound and could damage the camera if that happened too often)